The present invention relates to a method of producing metal by electrolysis in a molten salt bath.
The present invention may be thought of as an improvement on the practice of using a glass barrier in the manner set forth in U.S. Pat. Nos. 3,773,643 and 3,779,699 of A. S. Russell and L. L. Knapp for "Furnace Structure", issued respectively on Nov. 20, and Dec. 18, 1973. These patents point out that it can be extremely difficult to contain the molten salt baths used for producing metal by electrolysis of aluminum chloride in bipolar cells. It has been discovered that, while the glass barrier will usually effectively contain molten salt baths, it is nevertheless possible occasionally for the baths to leak through the barrier, for instance around the edges of the individual glass sheets making up the glass barrier, or through cracks arising in the glass.
As indicated in these patents of Russell and Knapp, leakage of the molten salt bath into contact with the steel container of a cell can cause evolution of a substance such as chlorine at anodic locations. It is possible for this chlorine to quickly eat a hole in the steel. At cathodic locations, fingers of the metal being produced can grow from the steel inwards along the crack to lead to costly short circuiting of at least a portion of the bipolar cell. In this connection, it has been found in practice to be almost impossible to absolutely isolate the steel outer shell of a cell from being able to engage in the carrying of electrical current, once either the anode or cathode of the cell finds a way to the shell, for instance through molten electrolyte in a crack.